November 2, 2009
The high-powered fiber laser now can take on the work of the blanking press.
When you think of high-volume blanking, a coil processing line, a mechanical press, and blank-stacking equipment probably come to mind. A typical mechanical press may cost $1.5 million to $2.5 million for a bed size 11.5 to 15 feet long and the tonnage required to run parts at 12 to 25 strokes per minute (SPM). With this comes a massive pit for the mechanical blanking press, as well as a higher ceiling so the huge machine can be installed and maintained. Bolsters for quick die change require significant operational floor space, and even more floor space is needed to store various die sets that blank different parts running through the press.
Meanwhile the auto industry, that bastion of high-volume production, has changed. The industry now demands lower volumes and just-in-time production with minimal inventory to handle ever-more-frequent design changes. For blanking, this has made hard tooling a greater liability. Quick die change has helped presses change over quickly, making shorter runs more efficient. But it still doesn't change the fact that hard tools need to be changed out in the first place.
But what if there were no hard tools?
For many years CO2 laser cutting systems have dominated blanking for low-volume and prototype requirements. But until recently the laser simply wasn't cost-effective for high- volume blanking. Now, however, high-powered fiber lasers provide a long-overdue, viable alternative to the blanking press (see Figure 1 and Figure 2).
Ten years ago a 6-kW CO2 laser with 10 percent wall plug energy efficiency could cut 0.040-inch-thick steel at 98 feet per minute. Today a 5-kW multimode fiber laser requires about 20 kVA of electrical power and has a wall plug energy efficiency of about 25 percent—more than double that of a 6-kW CO2 laser. It can cut material 0.040 in. thick at 197 FPM. Such fiber lasers have beams with wavelengths in the 1-micron range, delivered through fiber optics. Fiber delivery eliminates periodic downtimes for mirror alignment and cleaning and variations in cut quality and speed.
The list of laser advancements goes on. Footprints have shrunk. A 4- by 2- by 2-ft. cabinet can house a 5-kW fiber laser, and all the ancillary equipment for beam operation, including chillers and dust collectors, can be mounted on a 10- by 5-ft. platform. Laser system CNCs have morphed into pulse-type controls to adjust the output power of the laser while managing multiple axes of motion. CAD/CAM software includes all major attributes of the cutting routine in one front-end program that addresses all laser cutting process setups.
High-strength steels, gaining popularity because they reduce vehicle weight while maintaining structural integrity, present challenges for a blanking press line. With their high yield and tensile strengths, these metals require high tonnages, larger presses, and frequent die maintenance, because the cutting die's blades can dull quickly. Quality issues also arise with mechanically cut edges in this material. As material hardness increases, so does the propensity of microfractures after the blanking die shears the metal. Even worse is the potential for microcracks to develop into major fatigue-failure splits while the panel is in use on a vehicle.
In these cases and others, laser blanking can provide an advantage. Using lasers eliminates the need for the blanking die that, depending on its complexity, can cost about $100,000 initially, with annual maintenance costs about 25 percent of the initial purchase price. Lasers also eliminate a press foundation pit, and certain laser blanking configurations eliminate the coil-looping retardation pit. The laser blanking setup also requires a smaller crane capacity for coil handling. No blanking dies also means no die storage or rolling-bolster die change areas—and, of course, no die changes.
Lasers make blank development and nesting more flexible (see Figure 3). Any change requires altering the laser cutting CNC software program, not a new blanking die. While a blanking press line prefers shorter progressions at high press SPMs and a high-speed stacker, laser blanking prefers long progressions with a flexible stacker capable of handling several parts at a time (see Figure 1). Producing blanks for draw-die tryout in forming presses is also easier. The laser blanking line can produce several stacks of parts and make small modifications to the blank size during draw-die tryout.
Today a single-head laser system is less expensive than a blanking press capable of producing blanks up to 13 ft. long from coils up to 9.2 ft. wide—though certain simple, single-laser systems may not achieve the throughput of traditional press blanking lines. For increased throughput, a laser blanking line is designed with multiple laser heads in a series or in parallel for a single coil line.
Some engineering scrap is unavoidable. Forming presses need binder material to hold the metal with the proper tribology to control metal flow as the punch descends into the die. This binder material and functional holes are trimmed before assembly and add to the engineering scrap.
However, typical engineering scrap for an automobile body-in-white averages a staggering 40 percent. The North American car industry consumes about 10 million tons of steel per year. Reducing this by just a few percent, through better material utilization, would substantially improve bottom-line profitability.
For instance, engineers design corner-angle segment features into blanks to keep the cost of the blanking die low. Not only do such corner angles use more material, they limit blank formability, thus calling for tricks such as enlarged binders, higher press tonnages, and draw-bead features in the binder of the forming draw die. Laser cutting does not require such tricks and, to the contrary, prefers corner radii with curvilinear features. Blank contours with adequate curvilinear features increase both laser cutting speed and quality, as well as the blank's formability (see Figure 4).
Laser blanking can use nests from wide coils and still yield improved material utilization (see Figure 5 and Figure 6). A typical nesting routine for a blanking die often involves using a narrower slit coil width and detaching, at most, two finished blanks per press stroke. Laser cutting enables nesting with a dozen finished blanks to be detached within each coil progression. This eliminates the need for coil slitting, essentially a value-subtracting step that wastes material.
Lasers also allow the nesting of two different parts of identical thickness on the same strip and in a variety of orientations. Specifications sometimes restrict the free orientation of a part in a nest with respect to the rolling direction; however, the advantage of nesting multiple parts per progression out of wider coils reduces material waste and material cost per part, because of the elimination of coil slitting.
Finally, common-line cutting, made possible with the laser, pushes material utilization further.
In laser blanking, conveyor lanes must support the coil strip while avoiding the laser. For simple blank profiles, conveyor lanes adjust to clear a path for the laser cutting. For complex blanks or systems requiring higher production rates, conveyor lanes dynamically extend and retract during the laser cutting process, as well as between partial feeds to produce the blanks progressively (see Figure 7). In such a system, a single 5-kW fiber laser can produce five to 10 parts per minute, depending on part size and material thickness, and more lasers can be added for greater throughput.
Progressive lines laser-cut blanks directly from the steel strip. A five-laser system can produce 28 to 32 small parts per minute (PPM), and large, body-side outer blanks at 12 to 18 PPM. The strip is rolled out at a constant velocity, and four of the lasers make most of the cuts required for the developed blank. Pinch rolls hold down and drive the strip at each end of the four-laser configuration. After the last set of pinch rolls, the fifth cutting laser makes the final cuts to separate the developed blank from the strip.
Such a setup can be integrated with traditional coil lines and stacking systems, and additional laser modules can be added as needed. In an alternative configuration, a pin conveyor can be used with the pinch roll or clamping mechanism to support the strip's underside. The pin conveyors present the material to the lasers in a way that separates the scrap, allowing scrap to be removed and collected efficiently.
Blanking lines with multiple laser heads automatically distribute cutting routines among the lasers to achieve maximum production rates. Some lines can accommodate up to 15 lasers. When one laser is taken down for maintenance, the front end automatically reprograms the lasers to operate at maximum capability with the remaining lasers, redistributing cutting paths and adjusting the line speed. The pinch roll cells allow for the strip to continuously pass underneath the laser cutting heads, or the strip can be started and stopped in a progressive manner.
Such progressive laser blanking systems have various potential configurations. For example, a five-laser system used in a series and in parallel can detach a master blank from the coil-fed section, feed it to a blank-fed station, which in turn detaches the final blanks (see Figure 8).
As new technology emerges, more applications will prove that lasers can create cost-effective alternatives to hard tooling. Increased tooling costs; high-strength materials; and demand for energy-efficient, lean, and flexible manufacturing are all driving advances with industrial lasers. It's only a matter of time before the laser moves from the low-volume arena—where it dominates as a primary cutting process—to the high-production world.
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